Context: Relining with heat cure denture base resin is time-consuming and the patient has to remain without dentures within this period. Recently, some autopolymerizing resins marketed as hard chairside reline systems with low exothermic heat allow the dentists to reline prosthesis directly in the mouth. However, the decision to use these materials must be based on physical properties such as dimensional accuracy that directly influences the accuracy of fit of the denture base.Aim: The aim was to compare the linear dimensional changes of two hard chairside reline resins with two laboratory heat cure resins at 3 times intervals after processing.SettingsandDesign: A stainless steel split mold (International Organization for Standardization 1567) was used for sample fabrication. Five measurements of the reference dimensions (AB and CD) were measured directly from the mold and the samples with a profile projector, and mean difference were calculated.Subjectsand Methods: Forty samples were fabricated by incorporating the split mold into first pour of denture flasks and packing each of the chairside reline resins ("Kooliner" and "Ufi Gel Hard") and laboratory heat cure resins ("Dental Products of India Heat Cure" and "Trevalon"). The mean difference in dimensional change at 3 times intervals (0 h, 4 days and 2 months) were calculated and subjected to statistical analysis.StatisticalAnalysisUsed: One-way ANOVA, RMANOVA and posthoc Tukey's tests.Results: All resins showed different levels of significant shrinkage (P < 0.001) after processing (T 0 ) ranging from −0.128 to −0.310 mm. After 4 days (T 1 ), there was significant shrinkage (P < 0.001) ranging from −0.168 to −0.296 mm. After 2 months (T 2 ), there was again significant shrinkage (P < 0.001) ranging from −0.018 to −0.216 mm. Chairside reline resins showed less dimensional shrinkage at each time interval than the laboratory heat cure resins.Conclusions: Hard chairside resins are dimensionally accurate than the laboratory heat cure resins.

How to cite this article:Tewary S, Pawashe KG. Evaluation of linear dimensional accuracy of hard chairside and laboratory heat cure reline resins at different time intervals after processing. Indian J Dent Res 2014;25:686-91

The function of removable prosthesis is related to fit of its base to denture bearing area. However, due to continuous topographic changing nature of residual ridges, relining is frequently advised and has been conventionally achieved with heat cure denture base resins which are time-consuming and need multiple appointments. Recently, autopolymerizing resins with low exothermic heat have been marketed as chairside hard reline systems. Many of these materials [1] and their properties [2],[3],[4],[5],[6],[7],[8],[9],[10],[11],[12],[13],[14] have been evaluated. However, the decision to use these materials must be based on dimensional accuracy that directly influences the fit of denture base.

Subjects and Methods

A stainless steel split mold measuring 50 mm diameter and 0.5 mm thickness [Figure 1] and [Figure 2] with reference points A, B, C and D similar to the circular mold system described for denture base polymers in International Organization for Standardization Specification no. 1567 was fabricated. [15] The stainless steel split mold was incorporated in the first pour of denture flasks (KaVo Dental Products; NC, USA) to facilitate fabrication of the samples. The material groups, product details, their type and powder/liquid ratio used in the study are illustrated in [Table 1]. The materials were mixed, packed and processed as per manufacturer's instructions. A total of 40 samples were fabricated [Figure 3] by considering normal distribution at 95% confidence interval.

Just after processing, the samples were retrieved from the mold, measurements were made (T 0 ) and then stored in distilled water at 37°C in an incubator (Tempo Industrial Corporation, Mumbai: No. V-252) for subsequent recordings to be made after 4 days (T 1 ) and 2 months (T 2 ). T 0 time interval was selected based on the fact that maximum shrinkage occurs during processing. [16],[17] T 2 time interval was contemplated since few reline resins [4],[18] have exhibited continuous shrinkage up to 60 days of storage in water. An intermediate period of 4 days (T 1 ) between 0 h (T 0 ) and 2 months (T 2 ) was randomly selected to assess the pattern of dimensional change. The measurements were made using the profile projector (Nikon V 12, Japan) at 20X optical zoom [Figure 4]. The reference points AB and CD on the stainless steel mold were also measured, and the mean obtained (45.814 mm) was considered as the baseline reading.

The null hypothesis (H 0 ) for the present study would be "there is no difference in linear dimensional accuracy of hard chairside and laboratory heat cure reline resins after processing." Verification of the accuracy and repeatability of the measurements was accomplished by performing five repeated measurements by a single operator between each reference points (AB and CD) of the samples. The distance between the two ends was measured for evaluating the linear dimensional change, and mean was calculated for each sample. The difference between the mean dimension of each sample and the baseline reading on the stainless steel mold (45.814 mm) was calculated.

The values obtained were subjected to statistical analysis using one-way ANOVA, RMANOVA followed by post hoc Tukey's tests for multiple group comparison. Statistical results were interpreted with P < 0.001 as highly significant (HS); P < 0.05 as significant (S) and P > 0.05 as non-significant.

Results

[Table 2] and [Table 3] illustrate inter-group and intra-group comparison of the reline resins at 3 times intervals. Graph 1 elucidates the average linear dimensional change (mean difference from the baseline reading) exhibited by the samples at 3 times intervals.

Table 2: Inter‑group comparison of the reline resins at 3 time intervals

At T 0 and T 1 , the mean dimensional change was maximum for Trevalon followed by Dental Products of India (DPI) Heat Cure, Kooliner and Ufi Gel Hard. At T 2 , the dimensional change for the four groups was less when compared to T 1 , but remained maximum for Trevalon, followed by DPI Heat Cure. However, Kooliner exhibited less dimensional change in comparison to Ufi Gel Hard.

Removable dentures have been used extensively for the rehabilitation of completely and partially edentulous patients. The success of these prostheses greatly depends on the retention, support and stability. The major objective in the fabrication of complete denture and distal extension partial denture is to attain a denture base that conforms to the supporting tissues with a high degree of accuracy. With greater accuracy of the base in relation to the underlying tissues, the prosthesis becomes more stable. [19] However, due to the inevitable continuous process of alveolar bone resorption, removable dentures may become loose and less retentive resulting in soreness, loss of vertical dimension of occlusion and poor function. Thus, removable dentures need frequent relining to improve retention, stability, oral health, and esthetics. [20]

The relining of complete dentures involves solving all these problems encountered in the dentures, except positioning individual teeth. [21] The primary reason for relining is to re-establish tissue support for denture base, and neglecting it can result in excessive load over the residual ridge resulting in increased resorption and damage to the abutment. [2] The reline techniques of the removable dentures have been classified as direct and indirect. Direct technique involves relining of the partial denture directly inside the mouth (chairside) and the indirect technique involves laboratory processing of the denture with conventional heat polymerizing resins. Denture base polymers have been widely used in the relining of removable dentures, and the properties of these materials have made them the best suitable choice for their use.

The autopolymerizing polymers have the inherent disadvantage of polymerization shrinkage, regardless of the processing procedure. When polymer and monomer are mixed and processed, there is a volumetric shrinkage of about 6% due to change in density from 0.94 to 1.18 g/cm 3 . Many reline materials and methods have been specifically developed to minimize this shrinkage and distortion.

Denture bases fabricated with autopolymerizing resins are less stiff, more brittle and initially contain more unreacted monomer. Anusavice et al. [16] has stated that autopolymerizing resins exhibit 3% to 5% free monomer whereas the heat polymerizing resins exhibit 0.2% to 0.5% free monomer. The autopolymerizing resins often cause a chemical burn on the mucosa, have poor color stability, porous reline with unpleasant odor and difficulty in removing and replacing the material if improperly positioned. [40] Other problems associated with these materials include sluggish flow, tissue surface deficits and poor handling. [41]

Relining with conventional heat polymerizing denture base resins is time-consuming, may cause accidental distortion of the framework due to laboratory procedures with the added demerit that patient has to remain without denture within this period. This is particularly important considering the physical and nutritional limitations of old aged complete denture wearers. Recently, some autopolymerizing resins with low exothermic heat and with improved properties have been marketed as chairside hard reline systems. These materials allow the dentist to reline the prosthesis by direct method. Further, they have been claimed to be "hard and permanent" in nature, suitable for long term usage.

Kooliner (GC America) is a hard, chairside reline that has been formulated for reduced exothermic heat. It can be completely self-cured in patient's mouth in approximately 10 min. Ufi Gel Hard (VOCO GmbH) is a PMA-based cold cure permanent hard denture reliner. It is quick and easy to use for direct relines. Being a hard material similar to denture acrylic, it can be trimmed and polished, in the same way, as conventional heat polymerizing denture acrylic resin. The conditioner provided with Ufi Gel Hard prepares the surface of the reline denture to ensure optimal bonding. All relines fabricated using the Ufi Gel system can be carried out myostatically or myodynamically.

In the present study, at T 0 , all the samples of each brand of resins had undergone significant shrinkage with different patterns of dimensional change for each material (P < 0.001) which could be attributed to the reason that the polymerization process which occurs as the free radicals open the double bonds of the methyl methacrylate to create a chain reaction in which the monomer attaches to the polymer's free radicals. [43] The smaller dimensions of the samples relative to the master die exhibit this well documented polymerization shrinkage. [2],[4] The results are in agreement with Powers and Sakaguchi [44] and Anusavice et al. [16] Powers and Sakaguchi [44] observed that heat cure and self-cure resins have, in general, a dimensional accuracy of −0.4% and −0.1% respectively and Anusavice et al. [16] have asserted the same as −0.53% and −0.26% respectively.

The heat polymerizing resins (III a and IV a ) showed more dimensional change just after processing (T 0 ) when compared with the hard chairside reline resins (I a and II a ). The statistical results are in agreement with a similar study [2] which also showed significant (P < 0.05) dimensional change just after processing but the dimensional change obtained by them were comparatively less than the present study. These results obtained are in agreement with the fact that low degree of polymerization achieved in autopolymerizing resins by the use of chemical activator as opposed to that generated by heat activation in heat cure resins causes less polymerization shrinkage in autopolymerizing resins. [4] The range of dimensional change for autopolymerizing resins just after processing are in agreement with the observations by Mirza. [45]

At T 1 , all the samples showed mean dimensions significantly less than that of the baseline reading. Dimensional change for Trevalon (IV b ) was found to be maximum followed by DPI Heat Cure (III b ), Kooliner (I b ) and Ufi Gel Hard (II b ). Heat polymerizing resins (III b and IV b ) showed expansion from T 0 to T 1 , and the hard chairside reline resins showed continued shrinkage from T 0 to T 1 . The expansion seen in laboratory heat polymerizing resins may be due to water sorption, which partially overcomes the polymerization shrinkage. But the autopolymerizing resins showed continued shrinkage due to conversion of high residual monomer content, which was left as a result of inadequate polymerization. [4]

At T 2 , the mean dimensions of all the samples were found to be less than that of the baseline reading (P < 0.001) in the order IV c > III c > II c > I c . Both the hard chairside reline resins (I c and II c ) and laboratory heat polymerizing resins (III c and IV c ) showed expansion from T 1 to T 2 . The following expansion could be due to water sorption, [31] which could be attributed to the reason that the diffusion of water molecules occurs between macromolecules, which are forced slightly apart. This separation renders the molecules mobile and the inherent stresses created during polymerization of the acrylic resin can be relieved with a consequent intermolecular relaxation. [46]

Though all the materials showed expansion, the resulting dimensional change is exhibited as net polymerization shrinkage. The hard chairside autopolymerizing resins showed less linear dimensional change at the 3 times intervals than the laboratory heat polymerizing resins.

These materials can be clinically applicable to reline prosthesis directly in the mouth thereby saving time and appointments. Dimensional accuracy of these resins is of considerable importance to successful definitive treatment for old debilitating patients considering their physical and nutritional limitations. Moreover, as opposite to conventional relining procedures, the patients need not be without dentures for any substantial time period. Since this is an in-vitro study within a limited period of time; long-term clinical studies based on other physical, mechanical and mechanical properties are recommended.

Within the limitations of this study, the following conclusions were drawn:

All the reline resins under study showed different levels of significant shrinkage after processing. However, they compensated for the initial processing shrinkage but achieved partial compensation only

Autopolymerizing chairside reline resins were found to be more dimensionally accurate than conventional laboratory heat polymerizing reline resins

Kooliner exhibited least dimensional change at the end of 2 months and hence was most dimensionally accurate among all the materials under study